The Battle of South Mountain, also known as the Battle of Boonsboro Gap, occurred on 14 September 1862 during the American Civil War when Robert E. Lee's Confederate Army of Northern Virginia was defeated by George B. McClellan's Union Army of the Potomac in a minor engagement which preceded a decisive battle, the Battle of Antietam, which resulted in the defeat of Lee's invasion of Maryland.
Background[]
During Robert E. Lee's invasion of Maryland, he divided his Confederate Army of Northern Virginia into two forces: Stonewall Jackson captured Harpers Ferry in present-day West Virginia as the rest of his army, commanded by James Longstreet, camped at Boonsboro. Lee intended to wait for Jackson to capture Harpers Ferry in order to secure his rear before advancing deeper into Maryland, but the Union general George B. McClellan discovered Lee's plans from the captured Special Order 191 and led his army west to attack the isolated parts of Lee's divided force. Major-General Ambrose E. Burnside commanded the right-wing of his army (consisting of Joseph Hooker's I Corps and Jesse L. Reno's IX Corps) and led it to Turner's Gap and Fox's Gap, while Major-General William B. Franklin's VI Corps and Major-General Darius N. Couch's IV Corps were sent to Crampton's Gap in the south. Major-General Edwin V. Sumner held the II Corps and XII Corps in reserve in the center. Lee soon discovered McClellan's intelligence coup, so he aborted plans to send Longstreet north and instead had Longstreet reinforce the South Mountain passes. However, when the Union Army attacked on 14 September 1862, their only opponents were Daniel Harvey Hill's five brigades.
Battle[]
The Battle of Crampton's Gap
At Crampton's Gap to the south, a 500-man portion of Major-General Lafayette McLaws' Confederate States Army division faced 12,000 Federal troops. However, Franklin spent three hours deploying his forces, deploying Henry Warner Slocum on the right and William Farrar Smith on the left. They seized Crampton's Gap and took 400 prisoners, most of them late reinforcements from Brigadier-General Howell Cobb's brigade.
Union soldiers fighting off a Confederate counterattack at South Mountain
Meanwhile, Hill deployed his 5,000 Confederate soldiers across two miles of terrain at Turner's Gap, and the Union Iron Brigade attacked Colonel Alfred H. Colquitt's brigade, driving it up back the mountain. However, Colquitt's brigade continued to defend the pass. Robert E. Rodes' Alabama brigade was forced to withdraw rather than become isolated, and, by nightfall, the Federals held the high ground, while the Confederates still held the gap.
The Union assault on South Mountain
At Fox's Gap, Reno's IX Corps attacked Thomas Drayton's brigade. Brigadier-General Jacob D. Cox's Kanawha Division secured much of the land south of the gap, and Lieutenant-Colonel Rutherford B. Hayes of the 23rd Ohio Infantry Regiment was wounded during the attack. However, Cox's men grew exhausted while pushing back the North Carolinians, and Confederate reinforcements were deployed to the gap. Reno sent forward the rest of his corps to press the attack, but John Bell Hood arrived with more Confederate reinforcements at the same time. Reno and the Confederate Brigadier-General Samuel Garland, Jr. were both killed in action.
At nightfall, Lee ordered his outnumbered forces to withdraw from South Mountain, providing a morale boost to the defeat-stricken Army of the Potomac. However, McClellan squandered his opportunity to defeat Lee's army, engaging in his usual inactivity, which led to the surrender of Harpers Ferry to the Confederacy and Lee's ability to unite his scattered divisions at Sharpsburg in time for the Battle of Antietam on 17 September.